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The event:send action generates an event that it is sent to a pico, and takes two arguments, the first being a map with certain expected keys, and the second being a string (optional) containing the host of the receiving pico.
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Answer: "co_id" is not one of the expected keys in the map (which is the first (and only here) argument to the event:send action). The author was thinking that this would send an attribute of that name as one of the event attributes. Instead, since there was no "attrs" key in the map, no attributes were sent. Since "co_id" was not an expected key, the action just ignored it silently. The corrected code is:
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